How do I back up Android contacts to Gmail?

Should happen automatically. But to check go to menu, settings, accounts, google and check that Contacts is among the items checked and (via wifi if you can wait) click on synch now (some versions of android and/or phone manufacturers need you to press menu before that to get to synch)
And please note, this is not backed up to gmail, but to google using your gmail account as it's reference. some people get a bit of a panic on over the difference of the two :/

Should happen automatically. But to check go to menu, settings, accounts, google and check that Contacts is among the items checked and (via wifi if you can wait) click on synch now (some versions of android and/or phone manufacturers need you to press menu before that to get to synch)
And please note, this is not backed up to gmail, but to google using your gmail account as it's reference. some people get a bit of a panic on over the difference of the two :/